This invention relates to a brake actuator having an improved stone shield which dampens the impact of the brake actuator piston rod against the end wall surrounding the opening through which the piston reciprocates.
A conventional brake actuator of the type used for heavy vehicles, including trucks, tractor trailers and buses, includes a brake actuator housing having an end wall, an opening through the end wall and a piston reciprocal through the end wall opening for actuating the braking system of the vehicle. The piston generally includes an enlarged piston head and a piston rod which extends through the end wall opening which is operably connected to the braking system of the vehicle. In a typical application, the free end of the piston rod is connected by a clevis to the arm of a slack adjuster, such that the piston rod moves in an arcuate motion to actuate the brakes. Large vehicles generally include a pneumatic braking system, wherein pneumatic pressure acts directly or indirectly against the head of the piston to drive the piston rod through the end wall opening actuating the brake. In a diaphragm-type brake actuator, the pneumatic pressure of the vehicle braking system reacts against a flexible diaphragm, driving the piston toward the end wall to actuate the brakes. In a piston-type brake actuator, the piston head is sealed against he internal surface of the housing, such that the pneumatic pressure of the braking system reacts directly against the piston head.
A stone shield is generally located within the brake actuator housing surrounding the piston rod which limits the entry of road debris in the brake actuator housing, including road debris, such as dust, stones, ice, etc. Such stone shields are generally formed of a resilient material, such as synthetic rubber, or may be formed of a harder material, such as high density polyethylene. In its simplest form, the stone shield is an annular disk having an opening which receives the reciprocal piston rod. The stone shield may also include an annular end portion which is received through the end wall opening of the brake actuator. Where the stone shield is formed of a resilient material, the stone shield may also provide some damping of the force of the piston rod against the end wall surrounding the end wall opening.
Brake actuators are normally mounted under the vehicle or trailer chassis on a bracket in a generally horizontal orientation of about 20xc2x0 to 40xc2x0 from horizontal. As will be understood by those skilled in this art, the brake actuating piston rod continuously impacts the end wall of the brake actuator surrounding the opening through the end wall or the tubular end of the stone shield which extends through the end wall opening as the vehicle encounters normal road conditions. However, when the wheels of the vehicle or tractor trailer are driven over a sudden change in gradient, such as a chuck hole, the impact force may be as much as 40 to 80 Gs, causing damage to the piston or the brake actuator housing. Further, conventional stone shields must be assembled in the brake actuator before attachment of the clevis to the end of the piston rod. There has, therefore, been a long-felt need to dampen the force or impact of the piston rod against the brake actuator end wall surrounding the end wall opening to reduce the resultant damage to the piston rod and the brake actuator housing. The brake actuator having an improved damping stone shield of this invention solves this problem without any substantial increase in cost.
As set forth above, this invention relates to a brake actuator having an improved stone shield which dampens the impact of the piston rod of a brake actuator against the end wall surrounding the end wall opening through which the piston rod reciprocates during normal braking of the vehicle . The brake actuator may be any conventional brake actuator such as used on heavy vehicles including diaphragm and piston-type brake actuators which include a housing having an end wall, an opening through the end wall and a piston reciprocal through the end wall opening to actuate the vehicle braking system.
The improved damping stone shield of this invention includes an annular body portion having an opening which receives the brake actuator piston and a first end portion extending at least partially through the brake actuator end wall opening, preferably having a diameter less than the brake actuator end wall opening, and formed of a resilient material, such as synthetic rubber. The first end portion of the body portion includes a generally annular groove spaced from the body portion opening generally surrounding the piston rod and damping the impact of the piston rod against the brake actuator end wall surrounding the opening. In the most preferred embodiment of the improved damping shield of this invention, the groove in the first end portion is tapered or bell-shaped in cross-section having a rounded bottom or end wall and the groove extends into the body portion more than halfway through the body portion providing excellent damping of the piston rod against the end wall of the brake actuator and substantially reducing damage to the piston rod and brake actuator housing. In the disclosed embodiment, the stone shield includes a radial flange portion, preferably integral with the annular body portion, which overlies an inner surface of the brake actuator end wall having a diameter greater than the end wall opening. In the most preferred embodiment of the improved damping shield of this invention, the radial flange portion is spaced from a second end of the annular body portion having a diameter less than the diameter of the first end portion and the annular groove extends into the body portion to adjacent the juncture of the radial flange portion and the second end portion. The annular groove through the first end portion of the annular body portion of the stone shield thus converts the impact load of the piston rod from a compressive force to a shear compression force, optimizing the load capacity of the resilient stone shield. The testing of this design of damping stone shield indicates that the improved damping stone shield significantly reduces or eliminates damage to the piston rod and brake actuator housing resulting from significant impact loads as described above.
The preferred embodiment of the damping stone shield of this invention is also preferably split through one side permitting easy installation of the damping stone shield over the piston rod following attachment of the clevis to the free end of the piston rod. In the most preferred embodiment, the opening through the body portion of the stone shield is arcuate in cross-section or hyperbolic, having rounded enlarged openings to accommodate arcuate reciprocal movement of the brake actuator piston. In the preferred embodiment, the outer edge of the radial flange portion of the stone shield is rounded or bullet-shaped in cross-section and the stone shield is enclosed by a generally cup-shaped metal retainer in the brake actuator housing having a generally vertical wall portion surrounding the outer edge of the radial flange portion of the stone shield. Thus, when the piston rod is driven laterally as described above, the outer edge of the radial flange portion of the stone shield first contacts the surrounding wall portion of the retainer, damping lateral movement of the piston rod and reducing damage to the piston rod.
Other advantages and meritorious features of the improved brake actuator having a damping stone shield of this invention will be more fully understood from the following description of the preferred embodiments, the appended claims and the drawings, a brief description of which follows.